FIG. 1a is a cross-sectional view of a wind turbine rotor blade 10. The blade has an outer shell, which is fabricated from two half shells: a windward shell 11a and a leeward shell 11b. The shells 11a and 11b are moulded from glass-fibre reinforced plastic (GRP). Parts of the outer shell 11 are of sandwich panel construction and comprise a core 12 of lightweight foam (e.g. polyurethane), which is sandwiched between inner 13 and outer 14 GRP layers or ‘skins’.
The blade 10 comprises a first pair of spar caps 15a and 15b and a second pair of spar caps 16a, 16b. The respective pairs of spar caps 15a and 15b, 16a and 16b are arranged between sandwich panel regions of the shells 11a and 11b. One spar cap 15a, 16a of each pair is integrated with the windward shell 11a and the other spar cap 15b, 16b of each pair is integrated with the leeward shell 11b. The spar caps of the respective pairs are mutually opposed and extend longitudinally along the length of the blade 10.
A first longitudinally-extending shear web 17a bridges the first pair of spar caps 15a and 15b and a second longitudinally-extending shear web 17b bridges the second pair of spar caps 16a and 16b. The shear webs 17a and 17b in combination with the spar caps 15a and 15b and 16a and 16b form a pair of I-beam structures, which transfer loads effectively from the rotating blade 10 to the hub of the wind turbine. The spar caps 15a and 15b and 16a and 16b in particular transfer tensile and compressive bending loads, whilst the shear webs 17a and 17b transfer shear stresses in the blade 10.
Each spar cap 15a and 15b and 16a and 16b has a substantially rectangular cross section and is made up of a stack of pre-fabricated reinforcing strips 18. The strips 18 are pultruded strips of carbon-fibre reinforced plastic (CFRP), and are substantially flat and of rectangular cross section. The number of strips 18 in the stack depends upon the thickness of the strips 18 and the required thickness of the shells 11a and 11b, but typically the strips 18 each have a thickness of a few millimeters and there may be between three and twelve strips in the stack. The strips 18 have a high tensile strength, and hence have a high load bearing capacity.
FIG. 1b illustrates in perspective view a spar cap 15a in isolation. The strips 18 of the spar cap 15a are of decreasing lengths moving from the lower-most strip 18 to the upper-most strip 18, and the ends 19 of the strips 18 are staggered along the length of the spar cap 15a. Each end 19 is tapered, so as to facilitate stress transfer between strips 18 in the stack and to reduce stress concentrations when the stack is overlaid with materials forming the inner skin 13.
The blade 10 is made using a resin-infusion process as will now be described by way of example with reference to FIGS. 1c and 1d. Referring to FIG. 1c, this shows a mould 20 for a half shell of a wind turbine blade in cross-section. A glass-fibre layer 22 is arranged in the mould 20 to form the outer skin 14 of the blade 10. Three elongate panels 24 of polyurethane foam are arranged on top of the glass-fibre layer 22 to form the sandwich panel cores 12 referred to above. The foam panels 24 are spaced apart relative to one another to define a pair of channels 26 in between. A plurality of pultruded strips 18 of CFRP, as described above with reference to FIG. 1a, are stacked in the respective channels 26. Three strips 18 are shown in each stack in this example, but there may be any number of strips 18 in a stack.
Referring to FIG. 1d, once the strips 18 have been stacked, a second glass-fibre layer 28 is arranged on top of the foam panels 24 and the stacks of pultruded strips 18. The second glass-fibre layer 28 forms the inner skin 13 of the blade 10. Next, vacuum bagging film 30 is placed over the mould 20 to cover the layup. Sealing tape 32 is used to seal the vacuum bagging film 30 to a flange 34 of the mould 20. A vacuum pump 36 is used to withdraw air from the sealed region between the mould 20 and the vacuum bagging film 30, and resin 38 is supplied to the sealed region. The resin 38 infuses between the various laminate layers and fills any gaps in the laminate layup. Once sufficient resin 38 has been supplied to the mould 20, the mould 20 is heated whilst the vacuum is maintained to cure the resin 38 and bond the various layers together to form the half shell of the blade. The other half shell is made according to an identical process. Adhesive is then applied along the leading and trailing edges of the shells and the shells are bonded together to form the complete blade.
The integration of the spar caps 15a and 15b and 16a and 16b within the structure of the outer shells 11a and 11b avoids the need for a separate spar cap such as a reinforcing beam, which is typically bonded to an inner surface of the shell in many conventional wind turbine blades. Other examples of rotor blades having spar caps integral with the shell are described in EP 1 520 983, WO 2006/082479 and UK Patent Application GB 2497578.
The CFRP pultruded strips 18 extend along the majority of the length of the wind turbine blade 10. Modern wind turbine blades may be in excess of eighty meters long, and so it will be appreciated that these strips are very long and heavy. The length and weight of the strips presents challenges relating to the manufacture of the blades, and relating to the handling and transportation of the strips. The present invention aims to address these challenges by providing a convenient method of manufacturing this type of wind turbine blade, and by providing apparatus for use in the method.